Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCS process a synthesis gas (syngas) comprising a mixture of H2 and CO is bubbled up as a third phase through a solid/liquid slurry in a reactor. The products of the HCS reaction are generally gaseous and liquid hydrocarbons. The slurry comprises the liquid hydrocarbon products of the synthesis reaction and the dispersed, suspended solids comprise a hydrocarbon synthesis catalyst, most commonly a catalyst known in the Fischer-Tropsch process family of catalysts and entrained syngas.
Reactors which contain such a three phase slurry are sometimes referred to as xe2x80x9cbubble columnsxe2x80x9d, as is disclosed in U.S. Pat. No. 5,348,982. The catalyst particles are typically kept dispersed and suspended in the liquid by the lifting action of the syngas bubbling up through the slurry and by hydraulic means. Mechanical means such as impellers and propellers and the like are not used, as they will quickly erode and cause attrition of the catalyst particles. One or more vertical, gas disengaging downcomers may be used as hydraulic means to assist in maintaining more uniform catalyst dispersion, by providing a vertical catalyst circulation in the slurry, as is disclosed in U.S. Pat. No. 5,382,748. The slurry liquid comprises the liquid hydrocarbon products of the HCS reaction and needs to be separated from the catalyst particles and removed from the reactor for further processing and upgrading to the desired end products.
It is considered to be desirable to effectively and more efficiently convert coal and natural gas into synthesis gas, and synthesis gas into highly valued hydrocarbons, such as diesel motor fuel with high cetane number, petrochemical feedstocks, and hydrocarbon waxes. It is well known that synthesis gas will undergo conversion to form the reduction products of carbon monoxide, such as hydrocarbons, at temperatures in the range of from about 350xc2x0 F. (176xc2x0 C.) to about 850xc2x0 F. (454xc2x0 C.) and under pressures in the range of from about 1 to 1000 atmospheres (about 101.3 kPa to about 101,325 kPa), over a fairly wide variety of catalysts. The Fischer-Tropsch process, for example, which has been most extensively studied, produces a wide range of products including waxy materials, oxygenates and liquid hydrocarbons, a portion of which have been successfully used as low octane gasoline. The types of catalysts that have been studied for this and related processes include those based on metals or oxides of iron, cobalt, nickel, ruthenium, thorium, rhodium and osmium with and without promoters.
In a Fischer-Tropsch slurry reactor, the syngas is reacted on a powdered catalyst to form liquid hydrocarbons and waxes. The slurry is maintained at a constant level by continuously or intermittently removing wax from the reactor. The main problem with wax removal is that the catalyst in the wax should be separated from the slurry and returned to the reactor so as to maintain a constant inventory of catalyst in the reactor. It is preferable that reactors are run at steady state, meaning the rate of production or products remains relatively constant. Removing the catalyst from the reactor can upset the reactor steady state, in that any fluctuations in the catalyst concentration can affect the rate and type of products made in the reactor. Also, in order to keep the catalyst losses within the required replacement rate due to deactivation, the clarified wax removed from the system should not contain more than about 0.25% catalyst by weight. Accordingly, there is a need for a filtration process in which a clarified wax can be removed from the reactor while limiting the effect on the reactor steady state.
The present invention encompasses a process for producing synthetic hydrocarbon products. This invention involves feeding synthesis gas to a Fischer-Tropsch reactor and allowing the synthesis gas to react with catalyst to form liquid synthetic hydrocarbon products. The liquid synthetic hydrocarbon products are separated from the catalyst particles by passing the liquid synthetic hydrocarbon products through a plurality of closed end cylindrical filters located within the reactor, such that the liquid passes through the filters and the catalyst particles conglomerate on the outside of the filters. Each filter comprises a porous reactor side (outer) metal cylinder, a porous product side (inner) metal cylinder, and a filter medium located between the porous reactor side metal cylinder and the porous product side metal cylinder.
As the liquid synthetic hydrocarbon product is removed from the filters, a portion of it is pumped into a pulse surge vessel, such that the pressure of the pulse surge vessel is higher than the pressure of the Fischer-Tropsch reactor. At some point, it will become desirable to remove conglomerated catalyst from the reactor side of the filters, i.e. from the porous reactor side metal cylinder. At that time, the synthetic liquid hydrocarbon product through at least one of the filters should be stopped by closing a valve on the liquid synthetic hydrocarbon product outlet line.
To clear the filter, a quick opening valve on an outlet of the pulse surge vessel is opened, sending high pressure synthetic liquid hydrocarbon product from the pulse surge vessel to a backflushing liquid inlet of the filter. This allows for synthetic liquid hydrocarbon to flow from the pulse surge vessel into the filter, and then out of the filter into the Fischer-Tropsch reactor so as to dislodge the conglomerated catalyst particles from the outside of the filters.
Generally, the porosity of the filter medium is for about 0.5 to about 100 micron (1 micron=1xc3x9710xe2x88x926 m) filtration, and the porosity of the product side and reactor metal cylinders is accomplished with about xe2x85x9-inch (about 0.31 cm) to about 1-inch (about 2.54 cm) holes. The hole size is limited in that if the porosity were too large the metal cylinders would not be able maintain their rigid, cylindrical shape and thus lose its structural integrity. In this invention, the porosity of the product side metal cylinder is greater than that of the reactor side metal cylinder. The holes of the inside, product side metal cylinders are preferably tapered, such that the diameters of the holes are larger on the product side of the cylinders than at the surface of the cylinders facing the reactor side cylinders. The inside openings of the reactor side metal cylinder holes are preferably substantially the same size as the opening of the surface of the product side metal cylinders facing the reactor side cylinders. The reactor side metal cylinder holes may be tapered as well, with the holes having a larger diameter at the outer surface of the reactor side metal cylinder than at the inner surface of the same cylinder, the surface facing the product side cylinder.
In general, this invention discloses a filtering process than can be used for separating liquids from solids in almost any application, not just for Fischer-Tropsch product/catalyst separation. The filter of the present invention is designed with the Fischer-Tropsch process in mind, but is applicable for many similar processes.
Finally, the present invention discloses a novel filter for use as described above and in the illustrative embodiments. This invention also discloses a novel method for making the filter. The steps for making the filter comprise heat expanding a porous outer metal cylinder, cold shrinking a porous inner metal cylinder, inserting a cylindrical filter medium into the expanded porous outer metal cylinder, and placing the shrunken porous inner metal cylinder into the cylindrical filter medium. After the above steps are done, the combination of the three elements should be brought to an intermediate temperature so as to cool the heat expanded porous outer metal cylinder and heat the cold shrunken porous inner metal cylinder. This will allow for a tight seal between the elements of the filter.